1. Field of the Invention
The present invention relates to a mixer receiving an analog signal and a digital signal and providing an analog signal obtained by mixing of the received analog and digital signals.
2. Discussion of the Related Art
FIG. 1 schematically shows such a mixer 10 receiving an analog signal SI1, and a digital signal SI2 corresponding to a series of digital messages, each comprising a number N of bits, provided at a frequency F1. Mixer 10 provides an analog signal SO. An example of application corresponds to a karaoke device for which digital signal SI2 corresponds to a musical background and analog signal SI1 corresponds to a voice signal provided by a microphone. Analog signal SO then corresponds to the superposition of the voice signal on the music background, and can be used to control a loudspeaker.
A first conventional example of a mixer converts digital signal SI2 into an analog signal and adds the obtained analog signal to analog signal SI1 to provide analog signal SO. For an audio application, analog signal SO may be intended to control a class-D amplifier which drives a load, for example, a loudspeaker. However, a class-D amplifier is generally designed to be controlled by a pulse-width modulated analog signal (PWM). Although a pulse-width modulated signal is a two-state signal, it is considered as an analog signal since the average of such a signal corresponds to an analog signal. A disadvantage of the first mixer example is that, since analog signal SI1 is generally not in the form of a pulse-width modulated signal, the sum of signal SI1 and of the analog signal corresponding to the conversion of digital signal SI2 does not directly provide a pulse-width modulated analog signal.
FIG. 2 shows a second example of a mixer 10 comprising an analog-to-digital converter 12 receiving analog signal SI1 and providing a digital signal SO1 corresponding to a succession, at frequency F1, of digital messages each comprising N bits. Mixer 10 comprises an adder 14 receiving digital signal SO1 on a first input and digital signal SI2 on a second input and providing a digital signal SSUM corresponding to a succession, at frequency F1, of messages each comprising N bits. Signal SSUM is provided to a digital-to-analog converter 16 capable of providing analog signal SO. The second mixer example has the advantage that digital-to-analog converter 16 can be easily defined for signal SO to be a pulse-width modulated signal, which is then capable of directly controlling a class-D amplifier. The second example of mixer 10 is thus particularly well adapted to an audio application.
Analog-to-digital converter 12 for example is a Σ-Δ converter which comprises an analog-to-digital conversion unit (A/D) 18 receiving analog signal SI1 and providing a digital signal SD1 corresponding to a succession, at a frequency F2 greater than frequency F1, of messages each comprising M bits, M being smaller than N and for example equal to 1. Signal SD1 is provided to a decimation and filtering unit 20 which provides digital signal SO1. In such a type of converter 12, signal SD1 is provided at a high frequency F2 with respect to final frequency F1 to reject the quantization noise outside of the useful frequency band, the decimation and filtering unit 20 especially enabling filtering this quantization noise and keeping the signal intact in the useful frequency band.
Digital-to-analog converter 16 for example is of the type comprising an interpolation unit 22 corresponding to an interpolation filter receiving digital signal SSUM and providing a signal ST corresponding to a succession, at a frequency F3 greater than frequency F1, of messages each comprising N bits. Signal ST drives a digital-to-analog conversion unit (D/A) 24 which provides analog signal SO, possibly in the form of a pulse-width modulated signal.
An advantage of the second example of mixer 10 is that it can be almost totally formed of logic components, and can thus be easily made in the form of an integrated circuit. Further, such a mixer is particularly well adapted to the provision of a pulse-width modulated analog signal. However, such a mixer 10 has a relatively complex structure since it comprises decimation and filtering unit 20 which is, for example, formed of filters arranged in cascade, each performing a running average and a frequency division. Such a mixer 10 thus requires a significant silicon surface area when made in integrated form.